This invention relates to antennas to broadcast VHF data from a differential GPS ground station to supplement GPS reception for aircraft landings and, more generally, to elliptically-polarized omnidirective phase-progressive sub-arrays, vertical array antennas including a plurality of such sub-arrays, and cut-and-bend dipoles usable in such sub-arrays.
Enhanced accuracy applications of Global Positioning System (GPS) signals, such as use in aircraft landing and local control operations, can be enabled by derivation and local broadcast of Differential GPS (DGPS) signals to permit in-aircraft correction of local and other errors inherent in basic GPS signals. These errors may include ionospheric, tropospheric and satellite clock and ephemeris errors. To provide such DGPS signals, accurate local reception of GPS satellite signals is followed by derivation and local broadcast of the DGPS signals.
For such GPS signal reception, antenna systems providing a circular polarization characteristic in all directions horizontally and upward from the horizon, with a sharp cut-off characteristic below the horizon are described in U.S. Pat. No. 5,534,882, issued to the present inventor on Jul. 9, 1996, which is hereby incorporated herein by reference. Antennas with such characteristics are particularly suited to reception of signals from GPS satellites.
For local broadcast of DGPS data signals at VHF frequencies (e.g., for FAA Local Area Augmentation System (LAAS) for VHF Data Broadcast (VDB) applications) different antenna performance is required. Particular antenna requirements and characteristics may include accurate and reliable omnidirective broadcast of elliptically polarized VHF data signals, with elevation gain uniformity. Signal fades caused by ground reflections must also be minimized.
Objects of the present invention are, therefore, to provide new and improved antennas usable for such applications, and antennas, dipole arrays and cut-and-bend dipoles having one or more of the following characteristics and advantages:
omnidirective elliptical polarization; PA1 omnidirective phase-progressive radiation; PA1 low VSWR VHF band coverage via double-tuned dipoles: PA1 optimized sub-array excitation for low elevation lobing; PA1 dipoles with isolated frontal conductor for double-tuned performance; PA1 frontal divided transmission line structure for double tuning, provided via frontal conductor; PA1 low cost cut-and-bend construction; and PA1 economical and reliable dipole construction consisting basically of only two sheet-metal strips. PA1 (a) left and right conductive L-shaped strips having (i) respective left and right parallel portions extending outward from the mast in parallel spaced adjacent relation and (ii) left and right arm portions extending laterally from the respective parallel portions, oppositely from each other and diagonally to horizontal, and PA1 (b) a conductive frontal strip extending in parallel spaced adjacent relation to a portion of the combined length of said left and right arm portions to form a frontal divided transmission line structure. The excitation arrangement is coupled to intermediate points along the parallel portions of the L-shaped strips of each dipole to provide omnidirective phase-progressive excitation of each sub-array, with (i) the middle sub-array having phase-progressive excitation of reference amplitude and phase, (ii) the lower sub-array having phase-progressive excitation of nominally 70 percent amplitude and plus 90 degrees phase rotation relative to the reference amplitude and phase, and (iii) the upper sub-array having phase-progressive excitation of nominally 70 percent amplitude and minus 90 degree phase rotation relative to the reference amplitude and phase.